Cucumber (Cucumis sativus) is a major vegetable crop worldwide and among the most important crop species in the Cucurbitaceae family. They are eaten as a vegetable, either raw, cooked, or made into pickled cucumbers. The more than 100 varieties produce oblong fruits ranging in size from small picklers to large slicers and from can range in color from yellow or brown to a dark green for the cultivated varieties. Modern cultivated cucumbers are typically seedless and while they are generally considered less nutritious than most other fruits, the fresh cucumber is a good source of vitamins A, B1, B5, B6, B9, C, and K, and minerals. Most greenhouse varieties produce fruit without pollination and are gynoecious with respect to flowering, (i.e. produce only female flowers).
The yield potential of greenhouse cucumbers is high since at the base of every leaf one or more flowers are produced that will develop into fruits. Fruits are harvested at market maturity which requires a uniform diameter throughout the length of the fruits. The harvest or marketable stage is generally reached 12 to 15 days after opening of the flower. Compared with many crops, cucumber reaches harvest stage rapidly. In fact, many varieties of cucumber are ready to harvest after 50-60 days from seeding. Cucumber vines bear fruit in abundance and when harvested on a frequent basis (every 2 to 3 days) and in particular before the fruits reach full maturity, the setting of new flowers is encouraged and the harvest period may have a duration of 10 to 12 weeks. A two-crop rotation is most common, although three-crop rotations are also employed.
Cucumber yields depend mainly on the length of the harvest period, the spacing of individual plants, the pruning practice employed, the available light, the prevailing temperature, the particular variety, and good nutritional and pest management. The number of plants to be grown in a given area of greenhouse is determined by light conditions and by the method of training of the plants. While leaf overlapping and shading by adjacent plants must be avoided, summer light conditions will allow a higher planting density than winter conditions. The plants may be trained vertically or in the shape of an umbrella. Planting density is generally 2 plants per m2. Considering these variables, an average plant may yield 10-50 cucumbers per plant per harvest period. During mid-harvest on an umbrella-trained crop yields may range from 0.5-1.5 kg of fruit per plant per week.
It is a challenge to modern cucumber breeders to improve the yield of current cucumber hybrids, in particular the yield expressed as kg of fruit/plant. Conventional breeding methods have thus far not resulted in significant improvements of crop yields. For instance, the average yield of pickling cucumber in the United States has almost doubled in the period form 1960-1980 due to improved cultural practices, and selection for yield and disease resistance. However, in the last two decades, no significant improvements have been achieved. One of several routes can be taken to solve this problem.
One method for improving yield is based on further improving nutrient and pest management. However, in modern controlled production environments, these parameters have usually been optimized.
Other methods involve improving the breeding lines. Plant breeders and in particular seed companies employ elite breeding lines, generally referred to as “elite lines” to provide a constant quality product. The elite lines are the result of many years of inbreeding and combine multiple superior characteristics such as high yield, fruit quality, and resistance to pests, disease, or abiotic stress. The average yield of these elite lines is generally much higher than the original wild (landrace) accessions from which many of the modern cucumbers are descendants. The elite lines are used directly as crop plant or can be used to produce so-called F1 or single-cross hybrids, produced by a cross between two (homozygous or inbred) elite lines. The F1 hybrids thus combine the genetic properties of the two parents into a single plant. An add-on benefit of hybrids is that they express hybrid vigour or heterosis, the poorly understood phenomenon that hybrid plants grow better than either (inbred) parent and show higher yields.
Backcross or pedigree selection is one method by which breeders add desirable agronomic traits to their elite breeding lines. The method involves crossing the breeding line with a line that expresses the desirable trait followed by backcrossing offspring plants expressing the trait to the recurrent parent. As a result, the selection of an individual as a parent in a breeding program is based on the performance of its forebears. Such methods are most effective in breeding for qualitatively-inherited traits, i.e traits which are scored positive or negative. However, many traits of interest to growers, such as yield, earliness and quality, are quantitatively inherited and have low heritability. In the absence of a suitable source of the trait, no improvements can be made.
Recurrent selection is an alternative breeding method for improving breeding lines and involves systematic testing and selection of desirable progeny followed by recombination of the selected individuals to form a new population. Recurrent selection has proven effective for improving quantitative traits with low heritability, such as yield, in cucumber. Recurrent selection however does not increase the genetic basis underlying the various traits in a breeding program, and its potential is therefore limited. Over time, only marginal improvement have been realized.
There is a need for an additional method to improve the yield of cucumber. It is an aim of the present invention to provide methods for producing cucumber plants having improved crop yield. It is another aim of the present invention to provide cultivated cucumber plants having improved crop yield.